There are several choices to make when deciding how to build your pneumatic connections. Depending on your application, the tubing material could vary widely. Which should you choose?
Connections must be made throughout your application. You have to connect the air compressor to the coolers and receiver tanks that store the air. From there, you connect to the filters that clean the air. Then you have to run lines from there to wherever the machine or workstation is located. These connections are often made using black pipe or aluminum, and are larger than the connections made at the machine or workstation.
The connections at the machine and workstation begin with the filter/regulator/Lubricator (FRL) assembly. These should be positioned after every drop from the main compressor line (some newer systems no longer require lubrication in the compressed air). The connection from this FRL to the pneumatic valve bank and the actuators beyond are often made using plastic or rubber hose. Plastic tubes are measured on the outside diameter (OD) and generally used up to ½” OD. Air hose is measured on the inside diameter (ID), and it is used for larger sizes and applications that require better durability.
Flexibility, Compatibility, and Cost
When deciding which type of plastic tubing to use, it is important to consider three factors; flexibility, compatibility, and cost. Flexibility is critical in pneumatic applications, which often require connections to be made in tight spaces between components that may be moving as the machine cycles. Tubing that is not flexible enough for the application can kink, leading to reduced flow and premature failure.
Compatibility means making sure the tubing chosen meets the pressure and temperature requirements of the application. It also means the tubing is compatible with the media it will encounter in the application. There is also the possibility of interaction with lubricating oils in the airstream, or external media such as cutting or wash-down fluid.
Tubing must also be designed to work with the fitting that connects it to the valves or cylinders. Different fitting styles require different types of tubing. Push to connect fittings require a harder wall to resist the gripping fingers, while barbed fittings often require a softer wall. To make a reliable connection, these things must be compatible.
Finally, you have to consider cost. It may not be critical for some applications – the amount of tubing used in an application is usually small – but performance is critical. The list below gives an outline of the advantages and drawbacks of different materials. This outline represents generalities or “rules of thumb.” Each application is different, and should be confirmed with the tubing manufacturer.
Polyethylene tubing was the most common pneumatic tubing for many years due to its low cost, flexibility, and color-coding options. It works in most push to connect fittings, compression fittings that have a plastic sleeve, and some barb fittings. This tubing will kink if bent too far, and is only rated to 80-125psi depending upon size. This material also has limitations in temperature and chemical compatibility. The wall of the tubing is softer that nylon tubing, and it can wear out and “walk” out of a push to connect fitting.
Nylon tubing is more expensive than polyethylene, but that price brings many advantages. It has a harder wall, so it works better in most push to connect fittings. Nylon also has a wider range of chemical compatibility, and a wider operating temperature window. It can also be purchased in varying wall thicknesses and durometers (hardness), providing higher operating pressures. Some can reach 200-400psi depending upon size. These advantages come with a cost, and in this case it is flexibility. Nylon tubing has advanced and is becoming more flexible, but it still requires more room for a bend than is often found inside of pneumatically actuated equipment. Available in many colors, Nylon’s natural color is near transparent, so it is often used in place of glass as a level gauge on tanks.
Urethane tubing provides the best flexibility in pneumatic tubing. The soft wall makes it supple and easy to use in tight routing. It is available in vibrant colors that look very good when matched to the colors on a machine. It is more expensive than polyethylene, but you would need to be buying a great deal of it before that became the deciding factor. The downside to urethane tubing is the softness of the wall. This means lower pressures and a potential lack of compatibility with push to connect fittings. For push to connect applications, it is best to choose harder durometer (90) tubing with a heavy wall. Heavier wall thickness can increase the pressure drop through the tube which may slow down an actuator.
Copper or Stainless Steel
Copper was the original material for pneumatic applications, but was replaced with the introduction of plastic and push to connect fittings. There are still some applications where the rigidity and durability of copper is an advantage. For extreme applications stainless steel tubing provides temperature and chemical compatibility beyond the capabilities of plastic. Stainless steel tubing combined with instrumentation style compression fittings is often worth the extra cost and effort in critical or destructive applications.
When considering the choices in pneumatic connections it is possible to use the STAMP acronym to guide your decision; Size, Temperature, Application, Media and Pressure. If each of these factors is considered correctly you should have a safe and reliable connection for your pneumatic system.
Visit Kaman Fluid Power to learn more about hoses and fittings available for every application, or contact your local Kaman representative to access their expertise in making the right choices for your business.
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